1. Field of the Invention
This invention relates to improvements in sealing devices and more particularly, but not by way of limitation, to a combination weather and vapor seal for covering the rim space between the inner wall of a storage tank and the floating roof thereof.
2. Description of the Prior Art
Petroleum products, such as gasoline and the like, are usually stored in large cylindrical tanks having a roof which is supported on or floats on the surface of the product contained within the tank. As the level of the fluid within the tank rises or falls, the roof rises and falls freely therewith. The outer diameter of the floating roof is normally smaller than the inner diameter of the tank, and the annular space between the outer periphery of the roof and inner periphery of the tank is commonly called the "rim space". If this "rim space" is not covered in some manner, the product stored in the tank may be contaminated by the ambient weather conditions, which is a detriment, and furthermore, vapors from the stored product may escape through the "rim space" resulting in a loss of the product. In view of the current world wide shortage of petroleum products, it has become more and more important to protect the stored products from contamination due to weather conditions and to reduce loss of the product through the escape of the vapors.
In the past it has been a common practice to provide a seal for the rim space which extends between the inner tank wall and the top of the roof, and is mounted in such a manner as to move up and down with the movement of the roof. One particularly effective prior art seal includes a flexible metal shield which is mounted to the upper perimeter of the roof and extends diagonally upwardly toward the inner wall of the tank. A yieldable wiper blade is mounted on the end of the shield for engaging and pressing against the inner wall of the tank. One disadvantage of this type seal is that the usual rim space is of a variable width dimension throughout the annular extension thereof, and as a result an effective or efficient engagement between the inner periphery of the tank wall and the yieldable seal means is difficult to maintain. In addition, when the wiper blade is in a minimum pressure engagement with the tank wall, the wiper blade may "hang up" momentarily at an inwardly protruding weld seam in the tank wall as the roof moves upwardly within the tank. The remaining portion of the wiper may move upwardly with respect to the "caught" portion thereof, which may result in a permanent deformation of the wiper blade. A still further detriment or disadvantage of this type seal is the fact that the metal shield is not a unitary continuous piece that extends completely around the rim space, but rather comprises a pluraklity of adjacent shield sections which lap or abut one another. The joints between the adjacent sections of the shield provide areas of possible escape of the vapors from the interior of the tank. Even though these losses may not be great, the current energy shortage coupled with environmental concerns increases the problem. Additionally, it is frequently found that these sealing devices permit the temperatures within the rim space to reach undesirably high levels, which accelerates the generation of vapors from the product contained within the tank. This creates a greater depletion of the stored product and renders containing of the vapors more difficult.
In order to overcome these particular disadvantages, an improved seal has been developed for the rim space of a storage tank as shown in the Kinghorn et al U.S. Pat. No. 4,161,358, of which I am a co-inventor. The sealing means shown in my prior patent includes a flexible shield having one end rigidly connected to an upper portion of a floating roof of a storage tank and a second end extending upwardly and outwardly toward an interior wall of the tank. A flexible wiper blade is connected to the extending end of the shield for engaging the inner surface of the tank wall. The combined shield and wiper blade extend from the upper portion of the roof to the interior tank wall at an acute angle with respect to the horizontal, and the length of the shield is greater than the average rim space dimension divided by the cosine of the angle of the shield with the horizontal. This particularly selected dimensioning of the shield provides an effective seal between the wiper blade and the interior wall of the tank even when the rim space varies with lateral motion of the floating roof. One embodiment of the sealing means shown in my prior patent includes a double seal means at the rim, spaced between the floating roof and tank wall to even further increase the sealing efficiency and reduce loss of vapors from the product contained within the tank. The present day environmental standards and energy concerns have placed great demands on the design of the sealing elements for these floating roofs, and it may be that all such floating roofs will ultimately be required to have a double sealing arrangement at the rim space.
One disadvantage of the sealing means shown in my prior patent is the manner in which the flexible metal shield means is secured to the floating roof. The connection between the shield and the roof structure is a metal-to-metal seal and whereas the engagement of the yieldable sealing member with the wall of the tank is efficient, there has been leakage at the connection between the metal shield and the roof itself, which may not provide a sufficiently efficient seal for meeting the stringent requirements now placed on such structures.